One well known form of angular momentum mass flowmeters utilize magnets positioned on the turbine and/or the swirl generating impeller elements of the flowmeter. Reference is hereby made to U.S. Pat. No. 4,301,276 issued Dec. 3, 1983, for an illustration of such a flowmeter arrangement. In flowmeters of this type, mass flow is measured by measuring the time interval necessary for the magnets to move through a displacement angle .theta. between the turbine and impeller elements. The displacement angle .theta. is a function of the fluid torque exerted on the turbine element by the angular momentum of the fluid being measured. Sensing coils are mounted on the housing of the flowmeter to produce an output signal whenever the magnets pass by and come into flux exchange relationship with the coils. However,such arrangements are both bulky and costly. A need therefore exists for a small, light weight flowmeter readout which make it possible to reduce the size and weight of the overall flowmeter. This is particularly critical where mass flowmeters are utilized in applications, such as aircrafts, for example, where weight and size are at a premium.
In addition to the size, weight and cost disadvantage of magnet and winding pick-offs they also perform poorly at low flow ranges because the attraction forces between the magnet and windings (which can exceed static bearing forces) can prevent start-up at low flows with high viscosity fluids (i.e., cold fuel). In addition, performance of flowmeters using electromagnetic pick-offs are vulnerable to EMI/EMP/and lightning effects.
Applicant has found, that the size and weight of the flowmeter may be reduced by providing an optical readout which utilizes a pair of optical fibers extending into and out of the flowmeter housing, A beam of radiant energy (which may include energy both in and outside of the visible spectrum)is introduced into the flowmeter by an input fiber. The beam illuminates and is reflected from one of the flowmeter rotating elements. The reflected beam is received by and brought out of the flowmeter by an output optical fiber. Each rotating element contains a base interrupting means which interrupts the beam of radiant energy between the input and output fibers during each rotation thereby permitting measurement of the phase angle .theta. established between the two rotating components due to the angular momentum of the fluid being measured.
An optical pick-off of this type is both lighter, less costly, and smaller than electromagnetic pick-offs. Furthermore, such an optical pick-off is not subject to EMI/EMP and lightning effects nor is it subject to the magnetic attraction effects which limit operation of flowmeters using electromagnetic pick-offs at low flow rates. A further benefit of the invention is that a single pick-off fiber pair is adequate to detect the "start" reference point as well as the "stop" reference point on the rotating elements whereas separate pick-offs are required in electromagnetic systems..
It is therefore a principal objective of the invention to provide a flowmeter utilizing an optical readout.
A further objective of the invention is to provide a mass flowmeter utilizing a fiber optic readout.
Still another objective of the invention is to produce a small, light weight flowmeter utilizing a fiber optic readout.
Further objectives and advantages of the invention will become readily apparent as the description thereof proceeds.